Background: Immunotherapy-based treatments had shown to be active in several solid tumors, including in selected subtypes of sarcomas. IMMUNOSARC (NCT03277924) is a phase Ib/II trial [from Spanish (GEIS) and Italian (ISG) sarcoma groups], that tested the combination of nivolumab (anti-PD-1 inhibitor) plus sunitinib (anti-angiogenic agent) in advanced sarcomas. Among the 65 soft-tissue sarcoma (STS) patients (pts) enrolled, 48% were free of progression at 6 months, meeting the trial’s primary endpoint. EZH2 is the catalytic subunit of the Polycomb Repressive Complex 2 and it has been described to play an important role in the transcriptional repression of genes involved in T-cell migration and T-cell-mediated anti-tumor activity. The aim of this study was to explore the value of EZH2 gene expression as potential prognostic biomarker of the activity of immunotherapy-based schemes. Methods: The expression of EZH2 was evaluated in 64 paraffin tumor blocks, by direct transcriptomics, using HTG EdgeSeq Oncology Biomarkers Panel (HTG Molecular Diagnostics, Inc.; Tucson, AZ, USA). Data was normalized with DESeq2 and the cut-off of EZH2 expression was calculated with MAXSTAT R package. Gene expression was correlated with progression-free survival (PFS) by RECIST, overall survival (OS) and clinical benefit (patients with response or stable disease vs patients with progressive disease as best response). Results: Among the 64 pts analyzed, 52 (81%) showed overexpression of EZH2, considering a cut-off of 570.15 read counts. Undifferentiated pleomorphic sarcoma (UPS) and epithelioid sarcoma were the subtypes with higher expression of EZH2 with a median of read counts of 1888.04 (n = 10) and 1261.79 (n = 7), respectively. The lowest expressions were observed in extraskeletal myxoid chondrosarcoma (ECM) and alveolar soft-part sarcoma (ASPS) with a median of read counts of 461.42 (n = 4) and 680.84 (n = 7), respectively. Low expression of EZH2 was associated with better PFS (16.8 months vs. 3.9 months; p = 0.001) and better OS (NR vs. 20.0 months; p = 0.006). Moreover, low expression of EZH2 was also significantly associated with a clinical benefit of the patients treated with nivolumab plus sunitinib [relative risk (RR): 13; 95% CI: 3.0-56.9; p = < 0.001). Conclusions: Low expression of EZH2 was associated with better outcome in advanced STS patients treated with immunotherapy-based schemes. These results might support the rationale for the combination of EZH2 inhibitors with immune-modulating agents for future studies.